The present invention relates to an IP multicast distribution method for simultaneously transmitting data to a plurality of recipient hosts belonging to a specific group, and relates particularly to an IP multicast distribution system for distributing, to a recipient host on a private network, streaming data using the UDP (User Datagram Protocol) packet form, a streaming data distribution method and a program therefor.
IP multicasting is available for streaming broadband data. IP multicasting allows to reduce the load on a distribution server and to effectively utilize network resources.
Most current IP networks are incompatible with IP multicasting. In order to adapt IP multicasting to such IP networks, for example, Japanese Patent Laid-Open Publication No. Hei 10-242962 discloses a gateway device for copying IP multicast packets and transmitting the copies to IP addresses of individual users. Hereinafter, such gateway device is referred to as a mulficast/unicast conversion gateway, or a MU conversion gateway.
On the other hand, NAPT (Network Address Port Translation) is defined in RFC2662 “IP Network Address Translator (NAT) Terminology and Considerations”. Hereinafter, a gateway device with a NAPT function is referred to as a NAPT gateway. A NAPT gateway not only relates between a global IP address of a sender and a private IP address of receiver, but also relates between port numbers of the sender and the receiver on a transport layer.
A NAPT gateway may transmit streaming data to a reception host in a private IP address space. In this case, since UDP, as currently applied for a streaming technology, is a connectionless protocol, streaming data is transmitted without control information for its transmission. Therefore, the NAPT gateway does not obtain correspondence between a global IP address of a distribution server and a private address of a specific recipient host, and correspondence between the port numbers of the server and of the recipient host. As a result, the NAPT gateway cannot transfer the streaming data from the distribution server to the recipient host. Namely, a UDP packet cannot be transferred from the global IP space to a private IP space unless the port number and the address in the private IP space of the recipient host are designated at the NAPT gateway.
Further, a NAPT gateway can also be set so as to transmit all UDP packets from the global IP space to a specific recipient host in a private IP space. In this case, however, the NAPT gateway cannot transmit packets to one of the other recipient hosts.